Magnetic resonance imaging device, and determination method for high-frequency magnetic field conditions

Abstract

With an MRI apparatus using a transmission coil having multiple channels, a region desired to be diagnosed is efficiently imaged with high image quality. The MRI apparatus comprises a region setting means for setting a region in an imaging region, of which high quality image is desired to be obtained, as a first region, and an optimization means for determining at least one of amplitude and phase of a radio frequency wave transmitted to each of the multiple channels as a radio frequency magnetic field condition, and the optimization means determines the radio frequency magnetic field condition so that at least one of specific absorption ratio and signal value of a region that generates artifacts is not higher than a predetermined value defined for each, under a uniformity constraint condition that uniformity of radio frequency magnetic field distribution in the first region is not lower than a predetermined value.

The invention claimed is: 
     
       1. A magnetic resonance imaging apparatus, comprising:
 a transmission coil having multiple channels each for transmitting a radio frequency wave to a subject; 
 a region setter configured to set:
 a predetermined region in an imaging region as a first region, and 
 a region in the imaging region and different from the first region as a second region, which is a region including a region that generates artifacts or a region that shows high specific absorption ratio; and 
 
 an optimization module configured to determine at least one of amplitude or phase of a radio frequency wave transmitted to each of the multiple channels as a radio frequency magnetic field condition,
 wherein the optimization module determines the radio frequency magnetic field condition by optimizing a uniformity of radio frequency magnetic field distribution in the first region at a same time as optimizing at least one of specific absorption ratio or signal value of a region that generates artifacts, 
 wherein the optimized uniformity of radio frequency magnetic field distribution in the first region is not lower than a predetermined value, 
 wherein the optimized at least one of specific absorption ratio or signal value of a region that generates artifacts is not higher than a predetermined value defined for each, 
 wherein the optimization module uses at least one of the uniformity, specific absorption ratio, or signal value of a region that generates artifacts for the constraint condition, and at least one of the remainder for the objective function, calculates and determines the radio frequency magnetic field condition as a solution that optimizes the objective function, 
 wherein the constraint condition is that the uniformity is not lower than a predetermined value, and 
 wherein the objective function is such a function that, by minimizing a ratio of average of the radio frequency magnetic field in the first region and average of the radio frequency magnetic field in the second region, the signal value of the second region is made to be not higher than a predetermined value. 
 
 
     
     
       2. A magnetic resonance imaging apparatus, comprising:
 a transmission coil having multiple channels each for transmitting a radio frequency wave to a subject; 
 a region setter configured to set:
 a predetermined region in an imaging region as a first region, and 
 a region in the imaging region and different from the first region as a second region, which is a region including a region that generates artifacts or a region that shows high specific absorption ratio; and 
 
 an optimization module configured to determine at least one of amplitude or phase of a radio frequency wave transmitted to each of the multiple channels as a radio frequency magnetic field condition,
 wherein the optimization module determines the radio frequency magnetic field condition by optimizing a uniformity of radio frequency magnetic field distribution in the first region at a same time as optimizing at least one of specific absorption ratio or signal value of a region that generates artifacts, 
 wherein the optimized uniformity of radio frequency magnetic field distribution in the first region is not lower than a predetermined value, 
 wherein the optimized at least one of specific absorption ratio or signal value of a region that generates artifacts is not higher than a predetermined value defined for each, 
 wherein the optimization module uses at least one of the uniformity, specific absorption ratio, or signal value of a region that generates artifacts for the constraint condition, and at least one of the remainder for the objective function, calculates and determines the radio frequency magnetic field condition as a solution that optimizes the objective function, 
 wherein the constraint condition is that the uniformity is not lower than a predetermined value, and 
 wherein the objective function is such a function that, by maximizing average of the radio frequency magnetic field in the first region, the signal value of the second region is made to be not higher than a predetermined value. 
 
 
     
     
       3. A magnetic resonance imaging apparatus, comprising:
 a transmission coil having multiple channels each for transmitting a radio frequency wave to a subject; 
 a region setter configured to set:
 a predetermined region in an imaging region as a first region, and 
 a region in the imaging region and different from the first region as a second region, which is a region including a region that generates artifacts or a region that shows high specific absorption ratio; and 
 
 an optimization module configured to determine at least one of amplitude or phase of a radio frequency wave transmitted to each of the multiple channels as a radio frequency magnetic field condition,
 wherein the optimization module determines the radio frequency magnetic field condition by optimizing a uniformity of radio frequency magnetic field distribution in the first region at a same time as optimizing at least one of specific absorption ratio or signal value of a region that generates artifacts, 
 wherein the optimized uniformity of radio frequency magnetic field distribution in the first region is not lower than a predetermined value, 
 wherein the optimized at least one of specific absorption ratio or signal value of a region that generates artifacts is not higher than a predetermined value defined for each, 
 wherein the optimization module uses at least one of the uniformity, specific absorption ratio, or signal value of a region that generates artifacts for the constraint condition, and at least one of the remainder for the objective function, calculates and determines the radio frequency magnetic field condition as a solution that optimizes the objective function, and 
 wherein the constraint condition is that the uniformity is not lower than a predetermined value, and is such a condition that, by making average of the radio frequency magnetic field in the second region not higher than a predetermined value, the signal value of the second region is made to be not higher than a predetermined value. 
 
 
     
     
       4. A magnetic resonance imaging apparatus, comprising:
 a transmission coil having multiple channels each for transmitting a radio frequency wave to a subject; 
 a region setter configured to set:
 a predetermined region in an imaging region as a first region, and 
 a region in the imaging region and different from the first region as a second region, which is a region including a region that generates artifacts or a region that shows high specific absorption ratio; and 
 
 an optimization module configured to determine at least one of amplitude or phase of a radio frequency wave transmitted to each of the multiple channels as a radio frequency magnetic field condition,
 wherein the optimization module determines the radio frequency magnetic field condition by optimizing a uniformity of radio frequency magnetic field distribution in the first region at a same time as optimizing at least one of specific absorption ratio or signal value of a region that generates artifacts, 
 wherein the optimized uniformity of radio frequency magnetic field distribution in the first region is not lower than a predetermined value, 
 wherein the optimized at least one of specific absorption ratio or signal value of a region that generates artifacts is not higher than a predetermined value defined for each, 
 wherein the optimization module uses at least one of the uniformity, specific absorption ratio, or signal value of a region that generates artifacts for the constraint condition, and at least one of the remainder for the objective function, calculates and determines the radio frequency magnetic field condition as a solution that optimizes the objective function, 
 wherein the objective function is such a function that, by minimizing the sum of irradiation powers of the radio frequency magnetic fields transmitted from channels near the second region among the multiple channels, specific absorption ratio is made to be not higher than a predetermined value, and 
 wherein the constraint condition is that the uniformity is not lower than a predetermined value, and the specific absorption ratio (SAR) of the second region is not higher than a predetermined value.